Many modern aircraft are equipped with an airborne auxiliary power unit (“APU”) that provides electrical and pneumatic power to various parts of the aircraft for such tasks as environmental control, lighting, powering electronics, main engine starting, etc. In order to utilize an airborne APU various installation systems must be included with the APU. Some of the principle systems include mounts, inlet and exhaust, and pneumatic (bleed) and fire containment systems, including systems for detecting and extinguishing fires. FIG. 1 shows a typical airborne APU 100 installed in the tailcone of an aircraft (shown in dashed outline 105). Some of the key systems include an inlet system 110, a mounting system 115, a pneumatic system 120 and an exhaust system 125.
The exhaust system 125 performs many functions, including evacuating APU exhaust gas from the aircraft, attenuating exhaust noises, and providing compartment cooling airflow when used in conjunction with an eductor system. The functional design aspects of these duties are often integrated into a single exhaust muffler system to reduce weight. Unfortunately, the compromises between functionality, reliability, cost, and weight often result in an exhaust duct that will need repair work one or more times in its lifetime. The component of the exhaust duct that will most often need repair, or rather replacement, is the acoustic liner. This is because the thermal cycling and harsh environment that the acoustic liner is exposed to makes it rather brittle and subject to cracking. Because of potentially aggressive weight and cost targets, the design of an exhaust duct may not lend itself to simple and straightforward repair methods for replacing an acoustic liner. In addition, replacement of an acoustic liner typically requires removal of the APU 100 and exhaust system 125 from the aircraft, resulting in significant downtime for the operators of the aircraft.
Accordingly, there is a need for an exhaust duct and repair procedures that allow replacement of an acoustic liner on-wing, or from the outside of the aircraft, so that the useful life of the remainder of the exhaust duct can be extended. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background